I went and got my alternator tested today and it is no good so could that be the reason it doesn't do anything when you turn the key

 

Good possibility. Alternator keeps the car running and charges battery. If it does not work, the engine will still run by using battery reserve. Once it is gone, car will not start or run.

You can check voltage with a mutli-meter. With car running voltage at battery with a working alternator should be 14-15 volts. Voltage without car running and a charged battery should be 12 volts.

 

Bad batteries or batteries at the end of their life put a strain on the alternator. Alternator is meant to provide power while the engine is running and only providing a slight charge to the battery to replenish it from starting the engine. Low charge or dead batteries make the alternator work harder because it senses a lower voltage in the system. When your alternator is bad your'e best to replace the battery at the same time (unless it pretty new). Replace or fully charge your battery (trickle charge over night) and then have the alternator tested.